Over the past several decades air pollution control has been a priority concern of society. In the United States primary regulatory authority over industrial source air emissions resides in the U.S. Environmental Protection Agency (“EPA”). Over the years, the EPA has increased the stringency of its air pollution control programs, both by decreasing the limits on acceptable emissions and by continually increasing the number and types of regulated pollutants. The regulatory approach has been to force sources of air pollution to adopt the best available control technologies (“BACT”). In some instances, particularly where potentially toxic compounds are released into the atmosphere, sources are required to use the maximum available control technology (“MACT”). Thus, MACT must be used to control emissions of dioxins, furans and other substances regulated under the National Emission Standards for Hazardous Air Pollutants (“NESHAPS”).
In many instances, the types of pollutants emitted from an industrial source and the technologies available to control the pollution are highly dependent on the specific industrial process in use. EPA is currently undertaking a review of emissions of various air pollutants, such as volatile organic compounds (“VOCs”), semivolatile compounds (“SCs”), ammonia (NH3) and dioxins/furans, associated with various industrial processes, including the manufacturing of cement.
Control of pollution-created atmospheric haze is another element of EPA's air pollution regulatory program. Many areas of the U.S. have difficulty meeting regional haze limits, and EPA is investigating control strategies to achieve the regulatory goals. Ammonia emission is of concern in this respect because ammonia combines with nitric and sulfurous acid vapors to produce aerosols consisting of submicron salt particles which scatter light and reduce visibility. The ammonia-acid reactions may occur over time and at a great distance downwind from the point of release.
The foregoing problems are applicable to cement manufacturing facilities including those which use a precalciner prior to feeding the meal into the pyroprocessing kiln. A typical modem, prior-art cement manufacturing facility is shown in FIG. 1. While other dry and wet cement manufacturing processes are known, the dry, precalciner process depicted in FIG. 1 is now the most common and efficient.
The primary feed material, comprising a calcium-containing mineral used in manufacturing the cement, is obtained from a quarry, usually located nearby the cement plant. Typically the primary feed material is limestone, with smaller quantities of sand, clay, shale, and/or bauxite also being used. It has also become common to use industrial waste products, such as fly ash or slag, as feed materials. The feed materials provide the calcium, silica, aluminum and iron necessary to produce cement.
The quarried material is reduced in size by a crusher (not shown), and the crushed raw material is then transported to the cement plant, for example by motor or rail vehicle or by conveyor (also not shown). The proper proportions of the raw materials are then mixed and further reduced in size in a raw mill 10 to form a meal or feed material. For convenience the term “feed meal” is generally used herein to refer to the solid materials from the time they are processed in the raw mill to the time they enter the kiln. Thus, as used herein, feed meal includes the meal that has undergone precalcining.
The raw feed meal from raw mill 10 is then preheated in a preheating tower, comprising a series of vertically stacked cyclone chambers using exhaust gas from the kiln. While two such cyclone chambers (21, 22) are shown in FIG. 1, more (typically 3 or 4) may be used. Collectively these are referred to herein as the preheating tower and includes a precalciner 30. As depicted in FIG. 1, feed meal from raw mill 10 enters at the top of the preheating tower 21 and is preheated as it descends under the force of gravity.
The heated feed meal is then introduced into a precalciner 30, which converts the calcium carbonate (CaCO3) in the limestone (or other feed material) into calcium oxide (CaO), releasing a large amount of carbon dioxide (CO2) in the process. This is accomplished by heating the feed meal to high temperature—between about 1,650° F. (900° C.) and 1,800° F. (982° C.). The required temperature is higher than the temperature of the kiln exhaust gases, and so typically additional heat is generated in the precalciner by combustion of auxiliary fuel.
After precalcination the feed meal is introduced into a large rotary kiln 40 where it is heated to a temperature of about 2,700° F. (1,480° C.) to form “clinker,” consisting primarily of calcium silicates. Rotary kiln 40, which may be as long as 700 feet (213 meters), is substantially horizontal, with a slight tilt sufficient for gravity-assisted transport of the materials undergoing pyroprocessing along its length. Various fuels may be used to support combustion within the kiln in order to achieve the high processing temperature that is required. The hot clinker is then discharged from the kiln into a cooling chamber 50. After being cooled, the clinker is discharged from cooling chamber 50 and ground into fine particles. Normally, a small amount of gypsum is added during this final process stage.
The air used for combustion in kiln 40 first flows through cooling chamber 50, where it gains heat as it cools the clinker. The hot exhaust gases from kiln 40 flow through the precalciner 30 and then to the preheating tower 21 as described. After combustion in the kiln very little oxygen remains in the exhaust gas flow, and so additional air is introduced into precalciner 30 to support combustion. After passing through preheating tower 21, the exhaust gases are routed through raw mill 10 used to grind the raw feed materials, before being discharged into the atmosphere via stack 60. Because of the high particulate load, a baghouse or electrostatic precipitator 70 is used to remove particles from the gas flow, which are typical recycled back into and blended with the feed meal.
In FIG. 1 the movement of the solids (i.e., the feed materials, clinker, etc.) between the various processing operations is shown by solid lines, while the flow of gases is shown with dashed lines. It can thus be seen that the gas flow through the process is generally counter to the flow of the solids and, from the time the gases leave the kiln to the time they are exhausted into the atmosphere, they exchange heat with the feed meal, i.e., the gases are cooled as the feed meal is dried and heated. Thus, for example, the feed meal is progressively heated as it travels down the preheating tower from one preheating cyclone to the next, while the flue gases become successively cooler as they travel up the tower.
The feed materials used in the cement manufacturing process are inherently impure and vary depending on locale. The impurities include a large variety of naturally occurring organic substances, metals, etc. In addition, the raw materials used in the process typically contain various acid forming compounds, including sulfates, chlorides, nitrates, etc. Emissions from prior art cement plants include particulates, nitrogen oxides (NOx), sulfur dioxide (SO2), sulfuric acid mist (H2SO4), carbon monoxide (CO2), carbon dioxide (CO2), ammonia (NH3), hydrogen chloride (HCl), VOCs, SCs, metals, etc. As previously discussed, some of these emissions are considered hazardous and are subject to increasingly stringent regulatory controls. Unless proper controls are utilized, these may be released into the atmosphere during cement manufacturing.
The inventors have determined that a significant source of air pollutants from cement manufacturing is from the drying chambers used to reduce the moisture content of the feed meal prior to pyroprocessing in the kiln. As the feed meal is heated using the exhaust gases from the kiln, hydrocarbons with various boiling points fractionate, degrade and may be partially oxidized, such that the exhaust flue gases contain a range of aliphatic, aromatic and more complex organic species. These organic compounds in the exhaust are collectively referred to as either the total hydrocarbons (THC) or VOC, depending on the measurement technique used to quantify the emissions. The feed meal may also contain a mixture of organic and inorganic ammonium species that can decompose when heated to form HCl, sulfur trioxide (SO3) and NH3. Specific ammonium species may be released when heated by volatilization without decomposing, and these vapor phase emissions may condense to optically active submicron aerosols that scatter light, contributing to regional haze. Such species include, for example, ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), and ammonium bisulfate (NH4HSO4). Some of the problems associated with the presence of ammonia in the exhaust, and an approach to solving them, are discussed in the inventors' prior U.S. Pat. No. 6,060,030, the disclosure of which is incorporated by reference.
The inventors have determined that under certain conditions, HCl, chlorine (Cl2) and aromatic hydrocarbons such as benzene, react to from dioxins and furans. The number of isomers that may be formed is complex, depending on the gas temperature, the cooling rate, and the relative concentration of reaction species. Presently, it is known to reduce dioxin and furan formation by controlling the cooling rate of the exhaust gases. This approach is marginally effective and unpredictable.